Lifetime of catalysts

At present, intensive research is done to find optimal reaction conditions (concentration, temperature, pressure, flow rate, lifetime of catalyst, etc.) for different catalysts. 

Lifetime of catalysts can be prolonged by improvements in catalyst-deactivation-res istance adapted process design or regenerating methods, E mples for the industrial application of these concepts with different types of catalysts are presented. 

When the selectivity of a catalytic reaction is liable to be bad because of transport limitations, fixed-bed catalysts cannot be used with liquid-phase reactants. When selectivity is less important, fixed-bed catalysts have some advantages. Firstly, the catalyst need not be separated from the reaction products-a flow of reactants can simply be passed through the reactor. Furthermore, the catalyst can be readily thermally pretreated in a gas flow. Hence, deactivated catalysts can be regenerated in situ in the reactor. Exchange of the catalyst of a fixed-catalyst bed by another catalyst is, however, usually a tedious procedure. Fixed-catalyst beds are therefore used only within dedicated reactors in which only one or a limited number of products is produced. Also the lifetime of catalysts employed in fixed-bed reactors must usually be long, viz., two to five years. 

When 1-dodecene was passed over the catalyst at 300°C the product distribution shown in Fig. 5 was obtained. Within any one carbon number the ratio of branched chain/straight chain molecules was very high, being about 4 1 for C s and 3 1 for Cg s. The lifetime of catalysts was substantially reduced by cracking but as the cracking activity decreased the ability of the catalyst to skeletally isomerize without cracking became apparent. Thus, after 21 hours cracking of 1-dodecene at 300°C the products from the catalyst consisted almost entirely of branched (but unidentified) dodecenes. 

In the re-processing of materials not conforming to the specifications. Some other advantages are characteristic of particular areas such as the oil industry, where process analysers allow the lifetime of catalysts to be lengthened and coking and distillation flooding to be reduced. 

For metal chelate-mediated oxidation of sulfides, (salen)Mn systems such as 57," 58," ind 59" have been successfully exploited. A vanadium complex of 60 is also of some -tility. Certain multidentate ligands can prolong the lifetimes of catalysts" at the expense t enantioselectivity. 

The lifetime of catalysts is a key factor in the economics of many industrial processes especially when coke forms and deposits over the catalyst. It is therefore necessary to model the deactivation and regeneration of the catalysts utilized for the purpose of design, operation, control and optimization of these processes. 

Tables from 5 to 14 show, that the substitution of Al + and/or P + by divalent metal ions or tetravalent silicon in aluminophosphate structures creates both the Br0nsted and Lewis acid sites. These acid sites differ mutually in their donor-acceptor ability. The first one can transfer protons from the catalyst to the adsorbed molecules, whereas the latter can accept an electron pair from the adsorbed molecules. The strength and concentration of both types of acid sites determine the activity, selectivity, and lifetime of catalysts in acid-catalyzed reactions. The acid strength varies among aluminophosphates, and it is mainly dependent on the type of metal substituted in the framework. Also the catalytic performance is affected by structural characteristics of the framework such as the pore size, pore shape, or geometry.

For all three space velocities we plotted (Vf/Vp)2 versus t and found straight lines in agreement with eq. (13) from which we subsequently calculated the initial vanadium content, Vp°, the overall first-order rate constant, ky°, and the minimum lifetime of catalysts, Tmin- These were found to be constant within about 10 %, which is about the experimental uncertainty. We therefore conclude that the pore-mouth plugging model is useful as a tool to predict for instance Tsad at low space velocities from experiments at rather high space velocity. 

The utility of BFCs is determined largely by service life and power density. Accordingly, maximizing the lifetime of catalysts by effective electrocatalytic association with electrodes is essential to EFC development. There are numerous biochemical and biophysical facets of EFCs that can be optimized to enhance their efficiency and power, and many of those factors will be discussed and presented herein. 

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